This is such a subjective topic, it is brought up every year, and seldom answered very well. The quoted 10-20% of the total cost is usually pretty close. Break it down, as YOU see fit! Overseas, money is a lot scarcer and means more to the people you're spending time with. Here, too many of those providing their efforts expect and will almost demand a larger tip.
There are so many things take into consideration, and many mean a lot to the client, and many others don't matter at all. Some things to consider:
Is the guide working for the outfitter, and being paid a good wage or does he need your tip to get by? Did he earn it?
Is he providing all of his own equipment, vehicles, ATV's, gear or is it the outfitters?
Does he do anything extra for you, or simple drive around and look at easily seen and available game? Is he good at judging the quality of game and trophies you are looking for?
Are you getting room and board, livestock and services, packing, caping, skinning, delivery of meat to a processor?
Is he going out of his way doing anything extra for your comfort or peace of mind?
How much money are you fronting, how hard did you work and save for it, did you get tipped for earning it, and can you afford a large tip?
I went on one guided hunt, where I found my trophy myself, decided it was a great one, had to pack out the animal, cape it, and skin it out. After scoring in the record book, they wanted a bigger tip!!!!